Arc resistant molded members



Dec. 15, 1959 R. F. STERLING ARC RESISTANT MOLDED MEMBERS Filed Sept. 12, 1955 WITNESSES INVENTOR Robert F. Sterling.

United States Patent ARC RESISTANT MOLDED MEMBERS Robert F. Sterling, Churchill Borough, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 12, 1955, erial No. 533,569 6 Claims. (cl. 200-144 The present invention relates to molded thermoset resinous members adapted to be employed for use as electrical" insulation in close proximity to electrical arcs and has particular reference to insulating housing members for enclosing and supporting circuit breaker mechanisms and like electrical devices.

A common type of failure of molded resinous circuit breaker housings results from a track or conducting path forming thereon between two po-ints subjected to a high voltage stress such as the are drawn upon separation of the circuit breaker contacts. When repeatedly subjected to arcs, the molded resinous housings become carbonized. The carbon formed on the surfaces of the housing provides a path along which the arc may track across, thereby materially reducing or destroying the insulating properties of the housing.

Tests have been devised and are presently employed .for determining the resistance of various kinds of resinous compositions to this type of failure. One test for this property of arc'or tracking resistance is defined in ASTM Standard D495-48T.

There are certain materials which possess particularly high resistance to arcing or tracking as determined by this test. As an example, the literature reports that the arc resistance of molded melamine-formaldehyde resins is approximatelylOO to 180' seconds, depending on the fillers admixed therewith and the proportions used. This indicates that molded melamine-formaldehyde members may be. subjected to an electrical are under standard test conditions for 100 to 180 seconds before a low resistance or conducting track is produced on the surface thfi cof. Also, the literature reports that urea-formaldehyde resins have an arc resistance of from 100 to 150 seconds, depending on the fillers and their proportions blended with the urea-formaldehyde resins. Phenolic resins, on the other hand, have very poor are resistance values, ordinarily varying from seconds when embodying organic fillers and up to about 16 seconds when combined with mineral fillers.

It has been disclosed in copending US. application Serial No. 417,504, now Patent No. 2,806,109 issued September 10, 1957 assigned to the assignee of the present invention, that the are resistant properties of phenolic resins may be increased materially by embodying therein specific proportions of selected non-conducting solid inorganic fillers of predetermined particle size. The phenol-aldehyde resin, when so modified, has its are resistant properties increased to a degree such that it satisfactorily passes the ASTM test referred to hereinabove. The conditions of that test specify a current limited to a maximum of 40 milliamps at 12.5 kilovolts.

For many electrical applications, the low current conditions employed in the ASTM test are inadequate to evaluate completely and differentiate between insulating materials which in normal operation must withstand high energy arcs. For this reason a new and more severe test, described more fully hereinbelow, has been developed to" determihethe insulating characteristics of materials which are called upon to withstand high energy arcs. The newly developed test, which may be defined as a high power are or fuse wire test, utilizes a 500 ampere current with 500 volts between electrodes, the current being initially drawn by a 0.5 inch long piece of No. 24 copper wire applied to a surface of the material to be tested which copper wire fuses in about 0.1 second and results in a brief arc between the electrodes. The electrodes are reenergized in /2 second and if the material being tested carries the current the material is regarded as not satisfactory for applications where power arcs are liable to occur. Repetition of the test at intervals of 30 seconds at the same spot will eventually cause nearly all organics to eventually fail. The number of tests before the material becomes conductive is a measure of its relative resistance to power arcs. The tests will be set forth in detail hereinafter.

While melamine-formaldehyde resins and urea-formaldehyde resins and even the phenol-formaldehyde resin compositions, prepared as disclosed in application Serial No. 417,504, now Patent No. 2,806,109 issued September 10, 1957 are capable of withstanding the ASTM test for periods longer than seconds, they nonetheless fail the high. power are or fuse wire test, after a few repetitions, at best.

Various attempts have been made in the past to improve the are resistant properties of members molded from organic materials. To this end coatings of various kinds:

have been applied to the surfaces of the organic members: in the regions where arcs are drawn. Also, mechanical devices such as are chutes have been incorporated in. circuit breaker devices. While such attempts have met with varying degrees of success in improving the are resistant properties of the molded member, they also have had the undesirable efifect of increasing the cost of the finished device.

I have now discovered that molded resinous electrical devices such as circuit breaker housings which will withstand extremely high power arcs without tracking of the are along the surfaces thereof may be prepared from dicyandiamide-aldehyde molded resins embodying specific proportions of selected non-conducting solid inorganic particles of a predetermined size.

The insulating properties of molded resinous members prepared in accordance with this invention are such that the application of protective surface coatings and the inclusion in the device of arc chutes or like arc extinguishing structures may be omitted, thereby effecting a considerable reduction in cost of the device.

The object of the present invention is to provide molded thermoset resinous members adapted to be employed as electrical insulation in close proximity to high power electrical arcs which will withstand such arcs without undergoing material loss of insulating values as evidenced by the formation of tracking paths and the like.

Another object of the present invention is to provide dicyandiamide-aldehyde resin compositions embodying selected non-conducting solid inorganic fillers of predetermined particle size and in certain proportions whereby members molded therefrom have high power arc resistance.

Still another obiect of the present invention is to provide, in an electrical device having an electric current carrying member which develops an arc during operation of the device, a molded dicyandiamide-aldehyde resinous member containing certain non-conducting solid inorganic fillers of predetermined particle size having high power are resistance thereby permitting satisfactory operation of the device both during and after arcing.

A further object of the inventionis to provide a process for preparing molded electrical devices having high power arc resistance.

Other and further objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

In order to indicate even more fully the objects and capabilities of the present invention, reference is made to the following description taken in conjunction with the accompanying drawing wherein the single figure is a side elevation view of a circuit breaker, with the cover removed, embodying the principles of this invention.

In the attainment of the foregoing objects and in accordance with the present invention there is provided a molded thermoset resinous member, adapted to be employed as electrical insulation in close proximity to electrical arcs, which does not exhibit tracking or other material loss of insulating values. The member consists essentially of from 30% to 97% by weight of a thermoset dicyandiamide-aldehyde resin and from 70% to 3% by weight of intimately admixed fillers. At least 3% by weight of said member consists essentially of finely divided non-conducting inorganic refractory particles having an average particle size of not in excess of 5 microns.

Examples of non-conducting inorganic refractory materials in a particle size of from 0.05 to 5 microns which are suitable for use in the practice of this invention are silica, inorganic silicates, including zirconium silicate, calcium silicate, and aluminum silicate, zirconium oxide, aluminum oxide, calcium carbonate, and boron nitride.

Satisfactory moldings may be prepared from 97% to 30% by weight of dicyandiamide-aldehyde resin and the balance, from 3% to 70% by weight, of the finely divided inorganic refractory. Generally, the smaller the particle size of the refractory inorganic tiller and the larger the amount of filler employed with the resin, the higher the arc resistance of the final molded article. Refractory fillers in large amount, e.g., 75% and higher of the total weight of the member, render the molded article quite brittle and subject to cracking. The addition of other fillers in an amount of up to 120% of the weight of the dicyandiamide-aldehyde resin may be made to secure higher strength and impact values. Examples of suitable additional fillers for this purpose include chopped, ground, macerated or otherwise finely divided fibrous materials such as asbestos, wollastonite, glass fiber, cotton cambric, cotton duck, linen, wood flour and the like. The compositions also will ordinarily include a mold lubricant and a dye.

In preparing compositions to be molded into thermoset resinous members in accordance with this invention, a dicyandiamide-aldehyde resin, reacted in proportions and under conditions to produce what is commonly called a novolak, is initially prepared. The amount of aldehyde employed is so proportioned that from about 0.5 to 0.86 mol of aldehyde are used per mol of the dicyandiamide, the aldehyde generally being employed in amounts of about 0.80 mol per mol of dicyandiamide.

The novolak resins are thermoplastic solids in nature and require the addition of further amounts of aldehyde to render them thermosettable. Thus, additional amounts of aldehyde or methylene engendering substances are added in quantities suflicient to bring the ratio of di cyandiamide to aldehyde in the resin to at least 1:1.

In addition to using formaldehyde as the thermosetting agent, other sources of formaldehyde such as paraformaldehyde, furfuraldehyde, acetaldehyde and hexamethylene tetramine may be used.

In preparation of molded members, a given amount of dicyandiamide-aldehyde resin in powdered form is admixed with the required amount of thermosetting additive, for example, paraformaldehyde, a lubricant, such as calcium stearate, a dye, and one or more fibrous fillers such as wood flour. To this composition there is added at least 3% by weight, based on the total weight of the composition, of finely divided, inorganic refractory particles having an average diameter of not more than 5 microns. The entire mixture is then thoroughly blended, weighed portions are then placed in a suitable press and molded to members of desired shape and size at predetermined temperatures and pressures until the dicyandiamide resin reacts with the added aldehyde, and thermosets. Plates, tubes, baflles, and other insulating members are readily produced by such procedures. Meter bases, coil supports, contact supports and other apparatus may be molded from these compositions.

To illustrate even more fully the capabilities of the present invention, the following specific examples are set forth. The parts given are by weight unless otherwise indicated.

Example I There was introduced into a reaction vessel 10 mols (840 parts) of dicyandiamide, 8 mols (652 parts) formalin (37% aqueous formaldehyde), and 66 parts of benzene. The components were refluxed for 45 minutes, the benzene serving to keep the reflux at 70 C. at which to iperature the formation of methylolated dicyandiamide is favored. The benzene then was stripped from the mixture and the fluid remaining was refluxed for 2% hours at C. followed by dehydration under 26 inches of vacuum to a temperature of 55 C. The thermoplastic resin thus prepared was poured into a suitable container and permitted to solidify. Thereafter it was ground into a powder and milled on hot differential rolls with the following additives in the proportions indicated to form a molding compound having the composition:

Percent Resin 33.5 Hexamethylene tetramine 3.7 Calcium stearate 1.65 Aluminum silicate pigment 61.15

(Average particle size 0.5 micron) The powder was poured into suitable molds and heated to a temperature of C. to C. under a pressure of 2500 p.s.i. for a period of 8 minutes to form discs 4 inches in diameter and inch thick. A group of these discs when tested in accordance with ASTM Standard 13495-48"? did not fail for 185 seconds. Another group of these discs were subjected to the high power are or fuse wire test which comprised placing two vertical /2 inch diameter graphite electrodes /2 inch apart on the surface of the disc, the electrodes being connected to each other by a No. 24 copper wire. A current of 500 volts at 500 amperes direct current was applied to the electrodes and through the copper wire for 0.1 second, causing the wire to vaporize in the are generated, thereby breaking the current fiow. After an interval of /2 second, the voltage was reapplied on the graphite electrodes. Current did not flow between the electrodes thus demonstrating that the surfaces of the molded discs were nonconducting. The copper wire was replaced, the arc repeated and the voltage reapplied a number of times with a 30 second delay between each application. The discs prepared in accordance with the procedure of this example did not form a conducting path so as to pass the electrical current until after sixty reapplications of voltage after an equal number of vaporizatio'ns of the wire, thereby indicating the outstanding high power are resistance of the disc.

it is an important feature of this invention that certain proportions of phenol may be employed with the dicyandiamide in forming the resinous molded members of this invention. The presence of phenol in the resin renders the molded article more easily ejectable from the mold. The amount of phenol that may be added varies from 0.1% to an equimolar amount with the dicyandiamide. It has been found convenient, although not essential, when employing a phenol to first react the dicyandiamide with the aldehyde and then to add the phenol. The combined quantities of dicyandiamide and phenol reacted with the aldehyde must be such that there is a molar excess of dicyandiamide and phenol over the aldehyde. Examples of phenols which are suitable for use in vtit cordance with this invention include phenol, cresol, xylenolsghigher phenols. and mixtures o'fany'two or more of these; The following examples illustrate the preparatin.of.dicyandiamide aldehyde resins containing suitable proportions of. phenol.

, Example II.

There was introduced into 'a reaction vessel mols (420.p arts) ofdicyandiamide,8*mols -(652 parts) of formalin (37%? aqueous'formald'ehyde) and 66 parts of benzene?v The reactants were refluxed for a total of 40 m'inutespthe benzene serving to lower the boiling point of themix tnreto 170 C. Thereafter, 5 mols (538 parts) are %jaqueo.us phenol;were added to the methylolated diamide; The benzen'e: was stripped off and the mixture-refluxed for- 2 /z'hours'at95f' C. to 100 C. The product then 'was' cooled 'and' dehydrated under a vacuum ofj'2 7+ 28 inch'es 'ofimercury. toja final temperature of 86 C. Thetherrnoplastic material thus obtained then was "poured into suitable containers and permitted to ha'rd'emi after which: itwasgroundinto powder form. The resinpowder'then wasmilled on hot diflerentialrolls with the following materials in the proportions indicated tot form-La molding compound having the composition:

Percent Resin 1 t 33.5 Hexamethylene*tetramine* 3.7 Calcium*stearate*; 1.65 Aluminum silicate pigment 61.15

(Average particle size" 0.5 micron) 'aterialwas molded intodiscs'4 inches in diameter inch thick.- Whentested in accordance with the ASTM SteindardD'49S-48T test, the'discs didnot fail for wperiod of 'l88 seconds;- when subjected to the high power are o'r fuse wire test in-accordancc with the procedureddescribeddn 'Example' 'l,*a second group of discs withstoed' morethan ii -shots before current passed ac'rossftheir surface:

. Example IIIv e Percent Resinn 33.5 Hexamethylene tetramine 3.7 Calcium istearate-v... 1.65 Ahiminum silicate-piginentas 61.15

(Average particle size-0t5 micron) The; powder. was poured into suitable molds and heated to a temperature-of 150 C.'to- 185 C. under pressure .of 2 500 p. s.i-foraperiod of 8 minutes to form discs 4 inches indiameter and A; inch thick. A quantity of the discs thus prepared, when tested in accordance with ASTM Standard D495-48T, did not fail for 193 seconds. Another :quantity of thediscs, when subjected to the high power are or fuse wire test described inExample I, withst.ood more=-than 60 applications of current without any current passing across the surface of the disc.

6 ExampleIV Into a reaction vessel there wasintroduced 8 mols (672 parts). of dicyandiamide, 2 m0ls (220 parts) of aqueous phenol, and 8 mols (652 parts) of formalin (37% aqueous formaldehyde). The components were refluxed for 2 /2 hours at C. and dehydrated immediately thereafter under 26 inches of vacuum to'a temperature of C. The thermoplastic resin thus obtained was poured into a suitable container and permitted to harden. Thereafter itwas ground into a powder and milled on hot differential rolls with the following materials in the proportions indicated to provide a mold ing composition having the formulation:

(Average particle size 0.5 micron) This composition was poured into molds and'heated to a temperature of 150 to C. under a pressure of 2500 psi. for a period of 8 minutes to form a quantity of discs 4 inches in diameter and /3 inch thick. A quantity of the discs, when tested in accordance with the ASTM StandardD495-48T withstood 185 seconds without failing. A second' group of discs were subjected to the high power are or fuse wire test described in Ex ample I. These discs withstood 52 successive applica'-- tions of voltage before any current passed over their sur'-- face.

Referring to the drawing; thereis illustrated an elec-- trical circuit interrupter or breaker comprising generally a housing member 10 for receiving and supporting the several partsof the breakerstructure,a stationary con;- tact 12,1a movable. contact .14, an operating mechanism: 16 and a trip device 18. Member 10 is made from the composition of Example I.

Thestationary contact12 is rigidly secured to'the inner trip device 18, and is provided with a terminal connecting means, such as a screw 26, at its outer end for con necting thev circuit breaker in an electric circuit.

The movable contact 14-is rigidly secured on the free. end of a U-shaped switch member 28 having its legs 30. supported in recesses in the legs 32 of a U-shaped' opeerating lever '34 of molded insulating material. The: operating lever 34 ispivotally supported by trunnions 36 molded integral therewith and mounted insuitable companion openings in the housing member 10. An overcenteroperating spring 38 is connected under tension between the bight of the switch member 28 and a; releasablecarrier 40 pivoted on apin 42 supported in the housing 10; I

The operating lever 34 is provided with a handle 44 molded integral therewith and extending outwardly through an opening 46 in the housing 10. The operating: lever 34 isalso provided with an arcuate portion 48 which cooperates with the housing 10 to substantially close the opening46 in all positions of the handle. The switch member 28 is conductively connected by a flexible conductor 50 to one end of a bimetal element 52 forming? a part of the trip device 18 which is suitably secured to and supported by the inner end of theconductor strip 24.

The switch arm 28 is operated to manually open and close the contacts by operation of the lever 34 which is effected by manipulation of the handle 44. Movement of the handle 44 in a clockwise direction carries the pivotedends 10f the legs 30 of the switch member 28- across to theleft of the-line of action of the operating spring 38"which*then biases theswitch member 28' tothe open position and causes movement of the switch member 28 to the open position with a snap action.

The contacts are manually closed by reverse movement of the operating lever 34. Counterclockwise movement of the elver 34 from the open position to the closed position moves the upper pivoted ends of the legs 30 of the switch member 28 across to the right of the line of action of the spring 38 which in turn then acts to close the contacts with a snap action.

The circuit breaker is tripped open after a time delay in response to overload currents below a predetermined value, and instantaneously in response to overload currents above the predetermined value, or in response to short circuit currents, by means of the trip device 18. Operation of the trip device 18 causes release of the carrier 40 whereupon the operating spring 38 moves the carrier 40 clockwise about its pivot 42 until it is arrested by a projection 54 molded integral with the housing 10. The clockwise movement of the carrier 40 moves the line of action of the spring 38 across to the right of the center line of the switch arm 28 and the spring 38 then acts to move the switch member to the open position with a snap action.

The trip device 18 comprises the bimetal element 52 connected to the movable switch member by the flexible conductor 50 and electromagnetic means including a magnetic member or armature 56 rigidly mounted on the bimetal element 52 and a magnet yoke 58 movably supported in a recess 60 in the housing 10.- The conducting strip 24 extends along an end wall of the housing and is rigidly secured thereto by a screw 62, the bimetal element at one end being rigidly secured by suitable means, such as welding, to the upper inner end of the conductor strip 24. i

The armature 56 is rigidly secured to the bimetal element 52 by a rivet 64 which also rigidly fastens a latch member 66 to bimetal element 52. The latch member 66 normally engages and releasably restrains the carrier 40 in operative position.

Upon the occurrence of a low persistent overload current below a predetermined value of, for example, ten times normal rated current, the bimetal element 52 becomes heated, and when heated a predetermined amount bends toward the right causing the latch 66 to release the carrier 40 which effects opening movement of the switch member 28 in the manner previously described.

When an overload current above ten times normal rated current or a short circuit current occurs, the electromagnetic trip means is energized and the armature 56 is attracted toward the magnet yoke 58 bending the bimetal element 52 and causing the latch member 66 to instantaneously release the carrier 40 and open the contacts.

The releasable carrier 40 is reset and relatched and the contacts closed following an automatic opening operation by first moving the handle 44 clockwise to the off position and then counterclockwise to the on position. Movement of the handle to the off position causes the legs 32 of the operating lever to engage a pin 68 in the carrier 40 and move the carrier counterclockwise about its pivot 42. Near the end of this movement, the free or latching end of the carrier 40 Wipes by the latch 66, slightly bending the bimetal element which then resumes its normal latching position.

An arc chamber 70 is formed by the projection 54, the bottom wall 72 of the housing 10 and a projection 74 on the housing 10. The are gases are vented through a passage 76.

The properties and characteristics of this resinous circuit breaker are such that it prevents the formation or deposition of carbon on the surface thereof and thus prevents tracking of the are drawn between the breaker contacts. By using the molded resinous members of this invention, relatively inexpensive circuit breakers and like electrical devices may be produced since special surface coatings, are chutes, and other are extinguishing structures need not be employed. The elimination of such structures results in a material reduction in the cost of the devices.

While the present invention has been described with reference to particular embodiments thereof, it will be understood, of course, that certain changes, substitutions, modifications and the like may be made therein without departing from its true scope.

I claim as my invention:

1. A thermosettable molding composition consisting of (A) a resin derived by heating from 0.5 to 0.86 mols of an aldehyde with each one mol of dicyandiamide to provide a thermoplastic resin, (B) a compound selected from the group consisting of formaldehyde, paraformaldehyde, furfuraldehyde, acetaldehyde, and hexamethylene tetramine in an amount sufficient to raise the molar ratio of dicyandiamide to aldehyde in the thermoplastic resin to a ratio of at least 1:1, (C) a filler selected from the group consisting of finely divided fibrous fillers and inorganic refractory fillers, from 3% to 70% by weight of said composition being composed of finely divided nonconducting inorganic refractory particles having an average particle size of not in excess of 5 microns.

2. A thermosettable molding composition consisting of (A) a resin derived by heating an aldehyde, dicyandiamide, and a phenol to provide a thermoplastic resin, there being from 0.5 to 0.86 mols of aldehyde for each one mole of dicyandiamide and the phenol being present in an amount of from 0.1% to an equimolar amount with respect to the dicyandiamide, (B) a compound selected from the group consisting of formaldehyde, paraformaldehyde, furfuraldehyde, acetaldehyde, and .hcxamethyl tetramine in an amount sufficient to bring the molar ratio of the sum of dicyandiamide plus phenol to aldehyde in the thermosettable resin to at least 1: 1, (C) a filler selected from the group consisting of finely divided fibrous fillers and inorganic refractory fillers, from 3% to 70% by weight of said composition being composed of finely divided non-conducting inorganic refractory particles having an average particle size of not in excess of 5 microns.

3. In a circuit interrupter device having an electric current carrying member which develops an arc during circuit interrupting operation of the device and a molded electrically insulating member disposed adjacent to. the electric current carrying member and in close proximity to the point where the arc develops whereby the surface of the molded insulating member is heated and decomposed, the improvement which comprises forming at least a substantial thic (ness of the surface of the molded electrically insulating member of a thermosettable molding composition consisting of (A) a resin derived by heating from 0.5 to 0.86 mols of an aldehyde with each one mol of dicyandiamide to provide a thermoplastic resin, (B) a compound selected from the group consisting of formaldehyde, paraformaldehyde, furfuraldehyde, acetaldehyde, and hexamethylene tetramine in an amount sufficient to raise the molar ratio of dicyandiamide to aldehyde in the thermoplastic resin to a ratio of at least 1:1, (C) a filler selected from the group consisting of finely divided fibrous fillers and inorganic refractory fillers, from 3% to 70% by weight of said composition being composed of finely divided non-conducting inorganic refractory particles having an average particle size of not in excess of 5 microns.

4. In an electrical device having an electric current carrying member which is subject to development of an arc during operation and a molded electrically insulating member disposed adjacent to the electric current carrying member and in close proximity to the point where the arc develops whereby the surface of the molded insulating member is heated and decomposed, the im' provement which comprises forming the molded electrically insulating member of a thermosettable molding composition consisting of (A) a resin derived by heating an aldehyde, dicyandiamide, and a phenol to provide a thermoplastic resin, there being from 0.5 to 0.86 mols of aldehyde for each one mol of dicyandiamide and the phenol being present in an amount of from 0.1% to an equimolar amount with respect to the dicyandiamide, (B) a compound selected from the group consisting of formaldehyde, paraformaldehyde, furfuraldehyde, acetaldehyde, and hexamethylene tetrarnine in an amount sufficient to bring the molar ratio of the sum of dicyandiamide plus phenol to aldehyde in the thermosettable resin to at least 1:1, (C) a filter selected from the group consisting of finely divided fibrous fillers and inorganic refractory fillers, from 3% to 70% by weight of said composition being composed of finely divided non-conducting inorganic refractory particles having an average particle size of not in excess of 5 microns.

5. A circuit interrupter device comprising a housing of insulating material, terminals at opposite ends of said housing, and operating means disposed in said housing comprising separable contacts for drawing an arc, said housing comprising a molded thermoset resinous member derived by molding under heat and pressure, a thermosettable molding composition consisting of (A) a resin derived by heating from 0.5 to 0.86 mols of an aldehyde with each one mol of dicyandiamide to provide a thermoplastic resin, (B) a compound selected from the group consisting of formaldehyde, paraformaldehyde, furfuraldehyde, acetaldehyde, and hexamethylene tetrarnine in an amount sufiicient to raise the molar ratio of dicyandiamide to aldehyde in the thermoplastic resin to a ratio of at least 1:1, (C) a filler selected from the group consisting of finely divided fibrous fillers and inorganic refractory fillers, from 3% to 70% by weight of said com- 10 position being composed of finely divided non-conducting inorganic refractory particles having an average particle size of not in excess of 5 microns.

6. A circuit interrupter device comprising a housing of insulating material, terminals at opposite ends of said housing, and operating means disposed in said housing comprising separable contacts for drawing an arc, said housing comprising a molded thermoset resinous member derived by molding under heat and pressure, a thermosettable molding composition consisting of (A) a resin derived by heating an aldehyde, dicyandiamide, and a phenol to provide a thermoplastic resin, there being from 0.5 to 0.86 mols of aldehyde for each one mol of dicyandiamide and the phenol being present in an amount of from 0.1% to an equirnolar amount with respect to the dicyandiamide, (B) a compound selected from the group consisting of formaldehyde, paraformaldehyde, furfuraldehyde, acetaldehyde, and hexamethylene tetrarnine in an amount sufiicient to bring the molar ratio of the sum of dicyandiamide plus phenol to aldehyde in the thermosettable resin to at least 1:1, (C) a filler selected from the group consisting of finely divided fibrous fillers and inorganic refractory fillers, from 3% to by Weight of said composition being composed of finely divided non-conducting inorganic refractory particles having an average particle size of not in excess of 5 microns.

References Cited in the file of this patent UNITED STATES PATENTS 1,915,959 Barringer June 27, 1933 2,229,291 Groten et al. Ian. 21, 1941 2,645,693 Cole et al. July 14, 1953 2,684,344 Phillips July 20, 1954 

3. IN A CIRCUIT INTERRUPTER DEVICE HAVING AN ELECTRIC CURRENT CARRYING MEMBER WHICH DEVELOPS AN ARC DURING CIRCUIT INTERRUPTING OPERATION OF THE DEVICE AND A MOLDED ELECTRICALLY INSULATING MEMBER DISPOSED ADJACENT TO THE ELECTRIC CURRENT CARRYING MEMBER AND IN CLOSE PROXIMITY TO THE POINT WHERE THE ARC DEVELOPS WHEREBY THE SURFACE OF THE MOLDED INSULATING MEMBER IS HEATED AND DECOMPOSED, THE IMPROVEMENT WHICH COMPRISES FORMING AT LEAST A SUBSTANTIAL THICKNESS OF THE SURFACE OF THE MOLDED ELECTRICALLY INSULATING MEMBER OF A THERMOSETTABLE MOLDING COMPOSITION CONSISTING OF (A) A RESIN DERIVED BY HEATING FROM 0.5 TO 0.86 MOLS OF AN ALDEHYDE WITH EACH ONE MOL OF DICYANDIAMIDE TO PROVIDE A THERMOPLASTIC RESIN, (B) A COMPOUND SELECTED FROM THE GROUP CONSISTING OF FORMALDEHYDE, PARAFORMALDEHYDE, FURFURALDEHYDE, ACETALDEHYDE, AND HEXAMETHYLENE TETRAMINE IN AN AMOUNT SUFFICIENT TO RAISE THE MOLAR RATIO OF DICYANDIAMIDE TO ALDEHYDE IN THE THERMOPLASTIC RESIN TO A RATIO OF AT LEAST 1:1, (C) A FILLER SELECTED FROM THE GROUP CONSISTING OF FINELY DIVIDED FIBROUS FILLERS AND INORGANIC REFRACTORY FILLERS, FROM 3% TO 70% BY WEIGHT OF SAID COMPOSITION BEING COMPOSED OF FINELY DIVIDED NON-CONDUCTING INORGANIC REFRACTORY PARTICLES HAVING AN AVERAGE PARTICLE SIZE OF NOT IN EXCESS OF 5 MICRONS. 